When the pulp is diseased or injured and unable to repair itself, the pulp dies and infection set in at the root tip. The most common cause of pulp damage is a tooth fracture or a deep cavity that exposes the pulp to saliva. The bacteria found in saliva causes infection inside the tooth. Left untreated, infection eventually causes the pulp to abcess, thus swelling and pain. Infection can destroy the supporting bone that surrounds the tooth.

Trivia:

Tooth Decay is the 2nd most common disease in the U.S. after the common cold.